Interpretive Summary: Stimulation of plant productivity caused by Agaricus fairy rings has been reported, but little is known about factors aiding that productivity including soil aggregation and the beneficial microbial populations in the plant root zone. In collaboration with the Bureau of Land Management, Sidney, MT ARS scientists studied soils and plants in three concentric sampling zones caused by Agaricus lilaceps fairy rings in Eastern Montana rangeland. The study was initiated because these fairy rings are known to stimulate growth of western wheatgrass (Pascopyrum smithii), a desirable native forage. Results showed an increase in soil aggregate formation and stability in the plant stimulation zone of the rings compared to inside and outside the rings. Additional tests found high population size of fungi and high fungal activity in the stimulated zone. Furthermore, populations of soil-binding bacteria were also the highest in soil adhering to wheatgrass root collected in the stimulation zone. This study suggests that growth stimulation of western wheatgrass in these fairy rings may be attributed to the activity of the fungus which helped enhance soil quality in bulk soil at 0-15 cm depth and influenced the amount and functionality of specific beneficial bacterial communities in the grass root-adhering soil. Consequently, encouraging the growth of fairy rings could increase forage availability in Montana rangelands.

Technical Abstract:
Stimulation of plant productivity caused by Agaricus fairy rings has been reported, but nothing is known about soil aggregation and the microbial community structure of the stimulated zone, particularly the communities that can bind to soil particles. We studied three concentric zones of Agaricus lilaceps fairy rings in Eastern Montana that stimulate western wheatgrass (Pascopyrum smithii): outside the ring (OUT), inside the ring (IN), and adjacent to the fungal fruiting bodies (SZ) to determine 1) soil aggregate formation and stability, 2) the microbial community composition and the N-acetyl-ß-D-glucosaminidase activity associated with bulk soil at 0-15 cm soil depth, and 3) the predominant culturable bacterial communities that can bind to soil adhering to wheatgrass root. In bulk soil, macroaggregates (4.75-2.00 and 2.00-0.25 mm) and aggregate stability increased in SZ compared to IN and OUT. The ratio fungal to bacteria (FAME) and N-acetyl-ß-D-glucosaminidase activity were the highest in SZ, suggesting high fungal population and high fungal cellulolytic activity. A soil sedimentation assay performed on the predominant isolates from root-adhering soil indicated more soil-binding bacteria in SZ than IN and OUT; Pseudomonas fluorescent and Stenotrophomonas maltophilia were the most efficient in binding soil in SZ, whereas Bacillus spp. were more efficient in IN. This study suggests that growth stimulation of western wheatgrass in A. lilaceps fairy rings may be attributed to the activity of the fungus by enhancing soil quality of bulk soil at 0-15 cm depth and influencing the amount and functionality of specific predominant microbial communities in the wheatgrass root-adhering soil.